This invention relates to hydraulic control valves and more particularly to a self-depressurizing hydraulic control valve having a shutoff valve which automatically depressurizes the control edges of the metering valve to prevent leakage and consequent electrochemical erosion.
Hydraulic control valves used in high pressure hydraulic systems are known to be subject to various forms of erosion resulting from cavitation, the presence of abrasive particles in the hydraulic fluid, or the existence of an electrokinetic streaming current flowing between the hydraulic fluid and the valve body. These phenomena are thought to act independently and a valve may suffer simultaneous damage during operation from any number of them. It is also thought that the major cause of erosion in high pressure valves is due to the effects of electrokinetic streaming currents. The practical result of such erosion is excessive leakage in the valve, which may in turn result in the overworking and overheating of the entire system. Also, where the valve is controlled by a feedback system, excessive leakage may cause the system to become erratic and unstable.
Of particular interest herein are the high performance servo valves used in aircraft control systems. These valves frequently operate with pressures in the neighborhood of 4,000 pounds per square inch and must offer precise flow control and a high degree of reliability. With the introduction of phosphate ester-based hydraulic fluids in these systems as fire and explosion deterents, a noticeable increase has been observed in the erosion damage and maintenance requirements of such valves.
It has been long known that streaming currents may be produced by the flow of insulating fluids in regions where the fluid is subjected to acceleration. For example, steaming currents sufficiently large to produce sparks have been generated in oil during the filling of petroleum storage tanks. It has also been noted that where streaming currents exist, high concentrations of the ions of certain contaminants in the insulating fluid, such as chlorine, sulphur and phosphorus, have been measured.
A tell-tale sign of electrochemical erosion in a hydraulic valve is the erosion or removal of metal just upstream of a metering edge and a concurrent deposit just downstream of the edge. The erosion occurs primarily when the valve is nulled and a high pressure differential exists across the metering edges. Under these conditions, a small amount of fluid is prone to leak through the metering edges, and it will usually undergo high acceleration while passing between the edges because of the large pressure differentials. When this so-called "quiescent" leakage occurs, charged particles of certain contaminants, having a predominant polarity (either positive or negative) attach themselves to the wall of the valve. This concentration of charged particles creates a small electrical field, and particles of the opposite sign are attracted to the area. The oppositely charged particles usually form a loosely held layer over the area in achieving an electrostatic balance and this resulting arrangement of charged particles is known as an electrical double layer.
Since the fluid in the boundary layer immediately above the surface is in motion, particles in the loosely held upper layer are continually being swept downstream by the fluid, and since the fluid is accelerating, more charged particles are leaving the area than are arriving. The result is a deficiency of charged particles in the upper layer. The erosion of the surface is caused when metal ions of the same sign as the upper layer particles are drawn away from the surface by this electrostatic imbalance. The ions of both the metals and the impurities which are swept away tend to deposit themselves immediately downstream of a metering edge which creates an excess of charged particles in that region. As a result of this flow of charged particles, called a "streaming current," a potential difference is created between areas immediately upstream and downstream of the metering edge and an electrical current begins to flow between these two areas. As this erosion process continues, the metering edge will be progressively damaged and leakage will increase, eventually to the point that the valve must be replaced.
Observation of the streaming current phenomena and duplication of it in the laboratory have indicated that it is a function of the electrical conductivity of the fluid, the strength of the electrical double layer, the fluid viscosity, and the velocity gradient in the fluid boundary layer. Numerous attempts have been made to alter these factors so as to minimize the streaming current and, therefore, the erosion. Some limited success has been achieved by changing the electrical conductivity or by increasing the viscosity of the hyraulic fluid. Other experiments have dealt with the possibility of reducing the strength of the electrical double layer by the addition of certain dipole molcules such as water to the hydraulic fluid. This method is only temporarily effective because the dipole substance eventually becomes saturated and ineffective as the contaminant level rises.
The most obvious and successful solution to the problem has been to attempt to filter out the continuous ions from the fluid by passing it through various types of ion-absorbing materials such as molecular sieves or fuller's earth filters. It is not presently possible, however, even with the best known filtering techniques, to completely remove the contaminating ions from the fluid. Contaminants may be reduced to a very low level and the fluid may appear chemically clean by analysis, but yet it may still have a sufficiently high level of ionic contamination to be erosive.
It is also possible to reduce the streaming current by reducing the velocity gradient in the immediate vicinity of the metering edge. Since the velocity gradient is determined in part by the geometry of the valve near the metering edge, numerous attempts have been made to shape the valve so as to reduce the velocity gradient. A typical example is a design which causes the fluid pressure to drop in a number of steps across a number of metering edges rather than in a single sharp drop across a single edge. Despite the efforts in this area, no significant reductions in electrochemical erosion have been achieved by making changes in valve geometry.
It remains, then, that the best way presently known to minimize the effect of electrokinetic streaming current erosion is to reduce or eliminate leakage flow across the valve when it is closed. It is possible, of course, to design a valve with sufficient overlap between the slide and sleeve to practically eliminate leakage, but such a valve would have a relatively large "deadband" area and would lack the sensitivity necessary for use in an aircraft control system. One way to reduce leakage without sacrificing sensitivity is to block the high pressure supply and depressurize the metering edge when the valve is closed.
It is known in the art to shut off the high pressure flow to a valve in an aircraft flap control system with an electrically operated shutoff valve when the flaps reach the desired position, but such a system is relatively expensive, complex and heavy. It would be possible, of course, to build a system which sensed flap position and then mechanically shut off pressure to the control valve, but such a system would undoubtedly have the same drawbacks.
It is important in the design of a shutoff-type system for use in aircraft that increases in weight and complexity be minimized and that the shutoff feature be actuated only after the flaps have been accurately positioned. In an aircraft flap control system, as in many other hydraulic control systems, hydraulic pressure is not useful as an indicator of control position. System pressures may vary considerably as the flap approaches the desired position, depending upon the aerodynamic loads the system is required to overcome. The rate of flow of hydraulic fluid through the system, and particularly the control valve, however, may be useful as such an indicator because as the control approaches the desired position, the system automatically reduces the system flow rate by moving the valve toward a closed position. The subject of this disclosure is a control valve which contains a shutoff valve which operates in response to system flow rate and shuts off pressure to the metering valve when the system flow rate decreases to a preselected value.